Effects of Grass Carp in Midwest Reservoirs

Aquatic vegetation control is an ongoing issue for many waterways across the United States. Iowa\u27s landscape is naturally highly productive making this issue of nuisance levels of vegetation a common occurrence. Initial options for managing aquatic vegetation were to use mechanical or chemical methods but excessive cost and time necessitated the decision to use Grass Carp starting in 1973. In response to management concerns related to Grass Carp stocking into Iowa\u27s public lakes, recent changes in aquatic management approaches have been undertaken. Three lakes in Iowa representing different management scenarios in the use of Grass Carp as a management tool for aquatic vegetation were chosen. Red Haw Lake is void of Grass Carp, Mormon Trail Lake has a medium density of Grass Carp (9.5/ha) and Greenfield Lake has a high density of Grass Carp (33.5/ha). Aquatic vegetation and water chemistry were sampled in June-August 2007-2012. Benthic macroinvertebrates, young of year Bluegill and Largemouth Bass were sample in 2011 and 2012 to understand how changes in aquatic vegetation caused by Grass Carp change their communities and characteristics. Grass Carp can have a strong influence on vegetation diversity and abundance through their selective grazing. Varying densities of Grass Carp alters the intensity of their effect on the vegetation within the system. Benthic macroinvertebrate communities are changed by vegetation abundance. Characteristics of Bluegill and Largemouth Bass spawning activity is most closely linked to water temperature and the intensity to photoperiod. Young of year Bluegill and Largemouth Bass growth is effected by vegetation abundance but competition and food supply also have an influence on it. Vegetation management is not simply adding or removing Grass Carp. Often the issue is caused by more than this singular factor. Rather a more holistic watershed approach is required to achieve the desired result